The present invention relates to a method of accumulating and restituting cold, wherein, during cold-accumulating phases, in a storage vessel containing a mass of cold-accumulating and cooling liquid, a cluster of rigid aggregates of crystals of this frozen liquid is accumulated, and wherein, during cold-restitution phases, the cold accumulated in the storage vessel is restituted to a utilization circuit by fusion of said crystals in the vessel, by making a stream of said liquid circulate in closed circuit, successively through said cluster and said utilization circuit.
The present invention also relates to a device for implementing this method, including a storage vessel containing a cold-accumulating and cooling liquid, at least partially in the form of a cluster of rigid aggregates of crystals of said frozen liquid, these crystals being obtained by freezing this liquid by vaporizing a refrigerant brought into direct contact with cold-accumulating and cooling liquid, and means for injecting refrigerant at least partially in the liquid state, into this liquid.
The systems operating according to this known method, invented by M. L. Simon and described for example in Swiss patent No. 628,417, have multiple advantages over other known systems for accumulating cold wherein a cold-accumulating liquid, formed in general, as in the SIMON system, of water or an aqueous solution, for example a eutectic or non-eutectic solution of mineral salts such as sodium chloride or calcium chloride, is frozen on the outer surface of a refrigerant-evaporator or a heat exchanger traversed by water with glycol cooled to a temperature below 0.degree. C.
In particular, these new systems are notably more compact, simpler and more economical than other known systems.
Moreover, their thermodynamic efficiency, which constitutes an important quality coefficient, is superior to that of these conventional systems because, with this new process, the vaporization temperature of the refrigerant, which presents a large surface for direct contact with the cold-accumulating and cooling liquid to be frozen, is very close to the freezing temperature of this liquid, while with the other known systems, this vaporization temperature is several degrees Celsius less than said freezing temperature because the exchange of heat between the refrigerant and the cold-accumulating and cooling liquid is effected across the whole thickness of the solid ice deposit, of low thermal conductivity, which covers the above-mentioned evaporator or heat exchanger. This drawback is reduced, but not eliminated in other known systems wherein solid aggregates of macroscopic ice crystals are produced by indirect cooling of the accumulating liquid on the wall surface of a refrigerant evaporator and are mechanically scraped off or carried off by a thin film flowing by gravity over the surface of a cold wall to constitute with cooling liquid a heterogeneous mixture of pasty consistency which is conveyed and then discharged into a cold-storage vessel, as described in U.S. Pat. No. 4,480,455 or in U.S. Pat. No. 4,509,344.
Cold accumulating systems are in general characterized by two other economically significant quality coefficients: their cold-accumulating capacity per unit volume of space utilized by the installations (kcal/m.sup.3) on one hand, and their cooling efficiency of the cooling liquid during their cold-restitution phases on the other hand. This cooling efficiency may be defined, for a given flow rate D of the cooling liquid (m.sup.3 /h), as the ratio: R(D)=(.theta.1-.theta.2)/.theta.1-.theta.o) where .theta.1 is the temperature of the cooling liquid heated after its passage in the utilization circuit, on its arrival in the accumulator, where .theta.2 is the temperature of this liquid after its cooling in the accumulator, at the outlet of the latter, and where .theta.o is the freezing temperature of the cooling liquid. This ratio R(D), included between 0 and 1, is independent of the temperature .theta.1, but varies with the flow rate D. The product C..rho..D.R(D).(.theta.1-.theta.o) is equal to the power of cold extraction Pe (k.cal/h) of a cooling liquid of specific heat C and specific mass .rho.. For water (C.rho.)=1000 kcal/m.sup.3.
These two quality coefficients C and R(D) may be greater with the systems operating according to the method described in the above-mentioned Swiss patent than those of the other known cold-accumulation methods, at a lower cost. It is the analysis of the mechanisms responsible for the limitation of these quality coefficients which has led to the present invention.
According to the method described in the above-mentioned Swiss patent, it is customary to produce said crystals, as previously indicated, by the vaporization of a refrigerant in the bulk of said liquid, this vaporization being effected in a crystallization vessel or directly at the bottom of the storage vessel. This vaporization generates microscopic crystals which tend to be aggregated and, if one does not take special precautions, to be concentrated towards the top of the crystallization vessel or the storage vessel by decantation, to form there a slurry of ice crystals and accumulating liquid already having a certain solid or pasty consistency (commonly called "slurry") before said slurry is accumulated in the storage vessel to form said rigid solid cluster of ice commonly called "ice pack". In the other known above-mentioned methods with indirect freezing, for example that described in U.S. Pat. No. 4,480,445 or in U.S. Pat. No. 4,509,344, this slurry, formed of solid crystals of larger size, has a still less fluid consistency.
It is observed that by accumulating in the vessel this slurry containing coarse crystals and having an at least partly solid consistency, by discharging or letting it decant thereon, the ice cluster formed in the vessel has a porous microscopic structure but a heterogeneous and irregular macroscopic structure and a non uniform thickness and height. The bulk of the ice cluster frequently has cavities and an irregular network of communicating free spaces, of variable forms and sizes, which may attain several centimeters. These cavities and these free spaces are generally filled with gaseous refrigerant in the part of the cluster which emerges from the bulk of the cold-accumulating and cooling liquid contained in the vessel, and with this accumulating liquid and/or with gaseous refrigerant in the immersed part of said cluster.
Structural rearrangements accompanied by fissures may also occur in the bulk of the cluster in the course of formation (cold-accumulation) and by resorption of this cluster (cold-restitution) by the effect of mechanical tensions induced by gas pockets or defects of uniformity of the thickness and the height of this cluster, and/or induced by the development of forces of restrainment of said cluster by the walls of the accumulating vessel, or by elements solid with this vessel in the course of formation or resorption of said cluster.
The heterogeneous structure of the bulk of said cluster of ice and cold-accumulating and cooling liquid, and its non uniform thickness, limit the amount of ice which may be stored in a given vessel and, consequently, the cold-accumulating capacity of this vessel.
Moreover, the existence of a few spaces of large size in said cluster leads to the formation of preferential passages through this cluster. In the phases of fusion of this cluster by the reheated liquid, these passages tend to be spontaneously enlarged by fusion of the crystals at their surface, which rapidly induces hydraulic short-circuits through said cluster which, by diverting a considerable part of the cold-accumulating and cooling liquid out of the porous aggregate of the cluster, considerably lowers the effective surface of the contact interface between the liquid and the ice crystals where the exchange of heat takes place.
This results in a serious limitation of the cooling efficiency of the cold-accumulating and cooling liquid traversing said cluster. Moreover, on account of the random nature of the formation of said preferential passages, one sometimes observes fluctuations of the outlet temperature .theta.2 of the cooling liquid, hence of the cooling efficiency R(D), in the course of the same cold-restitution phase, or from one such phase to the other, and also variations of the maximum amount of cold which may be accumulated in the storage vessel.
The present invention has as a main object to increase the cold-accumulating capacity and the efficiency R(D) of cooling the cold-accumulating and cooling liquid of the systems operating according to the known new method of cold-accumulation. It also has the object of ensuring perfectly stable and reproducible operation of these systems.
The invention also has the object of enabling the cold-restitution to be effected with a higher flow rate D of cold-accumulating and cooling liquid than with the known systems of same dimensioning, hence to restitute the cold load accumulated in the storage vessel in a sorter period, at high power, while maintaining a high efficiency R(D), that is, by delivering the liquid at a temperature .theta.2 close to .theta.o.